1. Field of the Invention
The present invention relates to multi-microphone sound pick-up systems and, more particularly, to matching microphone sensitivity in multi-microphone sound pick-up systems.
2. Description of the Related Art
Suppressing interfering noise is still a major challenge for most communication devices involving a sound pick up system such as a microphone or a multi-microphone array. The multi-microphone array can selectively enhance sounds coming from certain directions while suppressing interference coming from other directions.
FIG. 1 shows a typical direction processing system in a two-microphone hearing aid. The two microphones pick-up sounds and convert them into electronic or digital signals. The output signal form the second microphone is delayed and subtracted from the output signal of the first microphone. The result is a signal with interference from certain directions being suppressed. In other words, the output signal is dependent on which directions the input signals come from. Therefore, the system is directional. The physical distance between the two microphones and the delay are two variables that control the characteristics of the directionality. For hearing aid applications, the physical distance is limited by the physical dimension of the hearing aid. The delay can be set in a delta-sigma analog-to-digital converter (A/D) or by use of an all-pass filter.
The sensitivity of the microphones of the sound pick up system must be matched in order to achieve good directionality. When the sensitivities of the microphones are not properly matched, then the directionality is substantially degraded and thus the ability to suppress interference coming from a particular direction is poor. FIGS. 2(a), 2(b), 2(c) and 2(d) illustrate representative polar patterns for microphone sensitivity discrepancies of 0, 1, 2, and 3 dB, respectively. Note that the representative polar pattern shown in FIG. 2(a) is the desired polar pattern which offers maximized directionality. The representative polar patterns shown in FIGS. 2(b)–2(d) are distorted polar patterns that respectively illustrate directionality becoming progressively worse as the sensitivity discrepancy increases respectively from 1, 2 and 3 dB. FIGS. 3(a), 3(b), 3(c) and 3(d) illustrate representative spectrum response for microphone sensitivity discrepancies of 0, 1, 2, and 3 dB, respectively, with reference to a 1 kHz pure tone in white noise. Note that the Signal-to-Noise Ratio of the spectrum shown in FIGS. 3(a)–3(d) is 14, 11, 9 and 7 dB, respectively. Accordingly, a good match of sensitivity between microphones is very important to good directionality.
Conventionally, manufacturers manually match the microphone for their multi-microphone directional processing systems. While manual matching of the microphones provides for improved directionality, the operational or manufacturing costs are substantial. Besides cost-effectiveness, manual matching has other problems that compromise manual matching. One problem is that microphone sensitivity tends to drift over time. Hence, once matched microphones can become mismatched over time. Another problem is that the sensitivity difference can depend on how the multi-microphone directional processing systems is used. For example, in hearing aid applications, a microphone pair that is perfectly matched as determined by measurements at manufacture may become mismatched when the hearing aid is put on a patient. This can occur because at manufacture the microphones are measured in a field where sound pressure level is the same everywhere (free field), while in real life situation (in situ) sound pressure may not distribute uniformly at microphone locations. Hence, when such pressure differences result, the microphones are in effect mismatched. In another word, because the microphones are matched in free field, not in situ, the microphones can actually be mismatched when used in real life, which degrades directionality.
Some manufacturers have used a fixed filter in their designs of multi-microphone directional processing systems. FIG. 4 illustrates a conventional two-microphone directional processing system 400 having a first microphone 402, a second microphone 404, a delay 406, a fixed filter 408, and a subtraction unit 410. The fixed filter 408 can serve to compensate for a mismatch in microphone sensitivity. The fixed filter approach is more cost-effective that the manual matching. However, the other problems (e.g., drift over time and in-situ mismatch) of manual matching are still present with the fixed filter approach.
Thus, there is a need for improved approaches to match sensitivities of microphones in multi-microphone directional processing systems.